In DragonVale for the iPhone and iPad, the River Dragon is an uncommon hybrid dragon with both Water and Earth attributes that you can breed once you're level 14. It has a base breeding time of 13 hours, sells for 900,000 coins, and earns 17,500 XP for successful breeding.

To breed a River Dragon, you just need to breed an Earth Dragon and a Water Dragon, or hybrids that contain both of these traits plus one other trait. Iceberg + Sandstorm seems to a popular combination.